Moving a site from a custom CMS to Silverstripe.

We've been asked by a client to move a site from a custom CMS service (Zeald.com - which looks like it is based originally upon Typo 3) to another CMS. Does anyone having tips on moving sites from one CMS to another (and if with this particular shift, all the better!). Our only method of accessing the data is to strip the rendered HTML/CSS using some script, so this is what we have to work with. The site owner is happy for us to build a site from scratch and leave the old archived in a folder so it is still accessible, but we'd like to give options.

We've taken a look through this forum, and have picked up some helpful tips but would like to be armed with as much information as possible before proceeding.